1. Field of the Invention
This Invention relates to an isocyanate/polyol casting resin and to its use as a binder for granular material for the production of open-cell moldings.
2. Discussion of Related Art
Open-cell moldings, for example of pebbles, are known. Thus, paving sets which allow rain to pass through are described in the Journal "ADAC Motorwelt", No. 2, 1993, pages 6, 7 and 8. In particular, there is a report there on a "filter set" made up of small pebbles which, in the same way as a sponge, allows rainwater to trickle through exactly where it falls. It is suitable not only for car parks but also for footpaths and bicycle paths.
Open-cell moldings of thermoset polyurethane are also known as a binder for a gravel-sand mixture. Thus, EP 468 608 describes a drainage element for lining wells. It consists essentially of 20 to 30 parts by weight of a defined aggregate and 1 part by weight of a polyurethane-based binder. Filter gravel with a particle size of 1 to 4 mm is used as the aggregate. Curing takes place at 80.degree. C. in the presence of a catalyst. The disadvantage is that grains of sand smaller than 1 mm in diameter have to be removed. In addition, curing at 80.degree. C. is too laborious and hence too uneconomical. Finally, the polyurethane binder is not evenly distributed. A particularly harmful aspect is that "nests" of unwetted fines are formed to the detriment of strength.
WO 92/18702 describes liquid-permeable elements which are produced by joining several unsupported segments with a cast elastomer. The segments consist of a mixture of gravel, polyurethane and casting resin. The grain fraction consists of particles 2 to 8 mm and 2 to 4 mm in diameter. The binder makes up 5% by weight.
In their prospectus entitled "Ecofilter", the Laubag company describes a pressure-resistant unsupported gravel filter produced from a polyurethane binder and a quartz sand mixture with a particle size of 1 to 4 mm. It is distinguished by the following advantages: high permeability to water, long-life well column, excellent filter effect, suitable to a depth of 200 m and high mechanical strength, more particularly high impact strength. Nothing is said about the production of the gravel filter or, in particular, about the polyurethane binder used.
DE 40 23 005 describes a polyurethane system which is suitable, for example, for filter construction, for fitting glass in window frames and for the production of sandwich elements. The polyol component contains 15 to 100% by weight of polyol, 0 to 85% by weight of filler, 0 to 5% by weight of a drying agent, such as zeolite, 0 to 2% by weight of thickener, 0 to 2% by weight of other auxiliaries, for example dyes or catalysts, and 1 to 10% by weight of a thixotropicizing agent of a mixture of polyamidoamine with a low molecular weight polyfunctional amine.
Starting out from this prior art, the problem addressed by the invention was to provide a binder for granular material which would provide for the safe and simple production of open-cell moldings of high mechanical strength which, in particular, are non-toxic.